Differential indicating means



D. w. ROPEfi April 12, 1932.

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Patented Apro 12, 1932 DENNEY W. ROPEJR, 0 F CHICAGU, ILLINGISDIFFERENTIAL INDICATING MEANS Original application filed January 6,1928, Serial No. 2%,025. Divided and this areplieation filed December19, 1930.

The art of electric generation and distribution has been constrained byeconomic m law to centralize generation as much as possible in order toincrease efficiency of production, increase reliability and continuityof service and to lower cost.

With the increased centralization of gen crating capacity the distanceof transmission must be extended to cover the required area in which thecorresponding consumer demand lies. Hence large amounts of power must betransmitted from such central sta- JH tions to out-lying points fordistribution.

' The most recent development in the art of electrical transmission anddistribution is to interconnect the generating stations of the samecompany or adjacent companies with long-distance h i gh-voltagetransmission lines. For economical reasons the voltage should be above100 kv. on most such installations. Through the rural district betweenthe cities, this transmission line is always 3n built as an overheadline because of the low cost as compared with an underground line, anduntil recently this has required that a substation be built at the edgeof the urban district in order to reduce the voltage from that of theoverhead line to a somewhat lower voltage for which underground cablecould be obtained. With the development of the so-called oil-filledcable, it is now possible to secure underground cable for operation at132 lrv. 3-phase, and eliminate the substation at the border of thecity. The elimination of this substation enables a marked economy to bemade in operation. In addition, the transformers located in thissubstation add to -15 drop in the line which must be compensated byadditional voltage regulating devices With the growth of the city, thesubstation might have to be moved in a few years, and with theoil-tilled cable this situation can be be handled by extending theunderground line Serial No. 503,519.

at a very moderate expense as compared with the cost of moving a largesubstation.

In order to secure insulation of the necessary quality, the oil used forsaturating the paper insulation and filling the hollow interior core ofthe conductor must remain fluid at all temperatures which will beexperienced during operation. The oil has a greater coeficient ofexpansion with temperature than the other portions of the cable so that,asthe temperature of the cable increases due to seasonal changes or dueto the load, the oil will be increased in volume to such an extent thata portion of it must flow from the hollow interior of the cable tosuitable reservoirs provided for the purpose, and these oil reservoirsmust be so located or devised that they will maintain an oil pressure onthe interior of the cable at all times.

Experiments have shown that, although the paper insulation whensaturated with a thin oil has a very high quality so that it will standabout double the stresses ordinarily used on high-voltage cable, thelife will be very short, that is, only a few hours if the oil is drainedfrom the interior of the conductor while the line is still in operationat normal voltage, a

in order to prevent the loss of oil from the entire line in case of acable failure or external damage it is desirable to divide the line intoa number or sections separated by joints which maintain the electricalconductivity of the copper conductors, but which prevent the oil fromflowing "from one section to another. Such joints are known as stopjoints. If the cable should "fail or the cable should be damexternallyso as to permit the oil to flow from the cable at a point near one ofthe feeding reservoirs, the oil would be drained out of the reservoir ina few hours. if the damage to the cable occurs at a longer distance fromthe feeding reservoir, the time is increased due to the resistance whichthe hollow conductor forms to the flow of the oil, In order to preventan electrical failure of the cable resulting from an oil leak, it isneces sary that the voltage be removed from the cable heifer the feedingreservoirs have be drained their oil; in the or She it is desirable thatthe voltage should be removed before the oil supply in the feedingreservoirs has been reduced to such an extent that the interior pressureof the oil at the terminal pothead is below the atmospheric pressure. Insuch a case aslight leak at the fittings of the terminal pothead wouldcause air and moistureto be sucked into the hollow'interior of thecable, making it necessary to replaceor reimpregnate this end section.The object of my invention is to provide a device which will give aprompt indication of an oil leak in the cable so that the necessarysteps can be taken to prevent the drainage of oil to such an extent thatthe replacing of a section on account of loss of oil will be necessary.

Experience has shown that this internal oil pressure develops leaks atthe potheads and at the joints which would not be troublesome with theordinary type of cable; and, further, that if these leaks receivedprompt attention repairs can be made in a few hours, whereas if theleaks were allowed to drain the oil from the reservoirs before beindiscovered, the repairs might take a few ays or a few weeks, dependingupon the circumstances and local conditions.

The fact that the cable when drained of oil will not stand normalpressure for more than a few hours before failure occurs makes itnecessary to have some devices which will indicate an oil leak and whichsupplement the ordinary electrical devices which disconnect a cable incase of electrical failure. The alternative is a very frequentinspection of the line, involving considerable expense. The largerportion of this expense can be avoided by the installation of thedevices covered by my invention.

In order to maintain the oil in the hollow interior of the cable and inthe feeding reservoirs in proper condition, the feeding reservoirs mustbe made so that they are sealed to prevent the ingress of air andmoisture. It is the usual practice to provide reservoirs made ofsuitable metal so that they will collapse like a bellows as the oil runsout, and this means that the head or pressure of the oil is maintainedpractically constant. The amount of oil in the reservoirs can, however,be determined by means of indicating devices attached to the collapsiblewalls of these reservoirs, or by having one end of the reservoirssupported on a spring or .other device which will in effect, weigh thereservoirs.

As the cables are actually installed, it is found expedient to have thefeeding reservoirs of such size that they are most economically made inseveral sections for convenience in handling. This makes it necessary todetermine the total amount of the oil in the several reservoirs that areconnected to one section of cable.

The three cables eachcontaining one conductor of the three-phaseunderground line are installed in the same conduit, and have theirterminals located as near as convenient where connection is made to theoverhead line. The three cables of one section of the line, whichsections are of the order of one mile in length, are, therefore,practically of identical length, and as the cables and reservoirs are ofthe same construction throughout, the amount of oil in the feedingreservoirs is adjusted so that it is practically the same for each ofthe three cables.

It is also the ordinary condition of threephase transmission over suchhigh-voltage lines. that the current is very closely the same on allthree conductors of the three-phase line. This means that the changes inthe temperature of the cable, due tothe load, will be practically thesame for all three cables. The changes in temperature of the cable dueto changes in the temperature of the ground with the varying seasonswill also be practically the same, as the three cables are, for otherreasons well known to the art, installed in adjacent ducts in theconduit. Under normal conditions, therefore, the changes in the amountof the oil in the feeding reservoirs connected to the three conductorswill be practically the same, and when the oil flows from one of thethree cables into its feeding reservoir it will also flow in the samedirection from the other two cables. If, however, one cable should bedamaged externally so as to develop an oil leak, the pressure of thehead of oil in its feeding reservoirs will cause the oil to fiow outthrough the leak so .that there will immediately be a graduallyincreasing difference of the amount of oil in this feeding reservoir ascompared with the other two. It is, therefore, quite desirable that thisdifference in the amount of oil in the three feeding reservoirsconnected to the three cables in one section of the line should bepromptly indicated in order to prevent the damage to the insulationwhich otherwise might occur, and it is a differential indicator of thischaracter which is covered by my invention.

III

According to the preferred form of my invention I provide differentialmeans for comparing the relative displacement of the insulating oil fromone cable with the displacement of oil from the other cables of the samesection to determine the above condition. For example, if afterstarting-with equal amounts of oil in the reservoirs the oil in onereservoir or group of reservoirs stands higher than the oil in thecorresponding reservoirs or groups of reservoirs for the other phases,either the first has more oil in it than it started with or the otherhas less. The former condition can occur if the corresponding section ofthe cable has risen in temperature above the temperature of the othercable and the oil has expanded into the reservoir. If the content ofrssassa the reservoirs increases substantially evenly or decreasessubstantially evenly normal operation is to be presumed. However, it theincrease or the decrease should be too great, warning should be given.

My method is a differential method, that is, I transmit a signal to thecentral station or other convenient location either in case there is anincrease or decrease of oil in the reservoir in one cable'or cablesection as compared with the other or others, or in case there is anabnormal increase or decrease in any one or all of the cables or cablesections.

The preferred way of doing this is to measure the quantity of oil in thereservoir or reservoirs of one phase or section of a phase and compareit with the quantity of oil in the reservoir or reservoirs of the otherphases or of the sections of the other phases. In the particular systemwhich I have herein illustrated I show' means which'is responsive to theweight of oil contained in the reservoirs, but it is to be understoodthat any other method of comparing amounts, as "for example, bycomparing levels or by comparing the relative expansion of the reservors, might be resorted to without departing from my invention.

This difierential method of comparing the quantities removes all errorscommon to the items compared, such, for example, as rise or fall oftemperature due to atmospheric or weather conditions and rise or fall oftemperature due to equal loading or unloading of the three cables in atransmission line.

In addition to the differential means for indicating the relativediiierence of oil contents, I provide maximum and minimum signalingmeans to indicate when an absolute maximum or absolute minimum at oilcontents of the reservoir for any one phase has been reached.

I also want to call attention to the fact that my invention isapplicable not only to oil filled underground cable, but to allconductors and apparatus employing fluid insulation .such as oil. Forexample, recently the art has taken up oil insulatedbusses compris ng aninsulated conductor disposed in an oil filled sheath. My invention isapplicable to such busses as well as the particular form of cable whichI have herein specifically described.

Because oil is the most suitable fluid insulation at present availablefor cables I have herein shown my invention as applied to a cableemploying this fluid insulation. lit is however to be understood thatthe invention as is likewise applicable to cables filled with otherfluid insulation it being merely necessary that the insulation used befree flowing, and this is a characteristic of all fluids. It is apparentthat the functioning of my invention is not afi'ected by thesubstitution of any oi clearness only a single phase;

Figure 2 is a d agram of connections showing the mechanism for comparingthe expansions of the three phases;

Figure 3 is a similar diagrammatic illustration of a modified form ofthe same; and

Figure 4 is a diagram of electrical connections used in connection withFigure 3.

Referring now to Figure 1, l have shown at 1 the single phase cableconstituting the conductor and the insulation for phase A. In a similarmanner the cables for phases B and C are indicated at 1 and 1". Sinceeach phase is the same as every other phase a description of one willsuflice for the others. The cable terminates in an oil filled pothead 3,as shown at the left side of the drawings, and at 4 in the right side ofthe drawings. The cable proper comprises a lead sheath, a hollowconductor, and wrapped insulation between the conductor and the leadsheath, the central part of the cable providing a canal or passagewayfor oil which keeps the wrapping completely. saturated with oil tomaintain the integrity of the insulation. Obviously the cable may beotherwise constructed without departing from my invention. The cable ismade up in suitable lengths joined at manholes, as indicated at 5, 5 andthe run of cable is sub-divided into sections by step joints asindicated at 6, 6, these stop joints being suitably constructed tosectionalize the oil channels of the cables. At one end or" eachsection, as for example, 88, standpipe connections communicating withthe corresponding section of the cable are provided. These standpipeconnections lead into reservoirs suitably designed and located at theadjacent ends of two sections. The pressure of oil in the cables may bemain tained by a suitable gravity pressure head or pneumatic pressurehead or other means for maintaining adequate pressure of oil to excludeair or moisture and insure free flow of the oil into the cable when thesame cools. Likewise, the potheads 3 and at are provided with standpipeconnections 77 for maintaining theend sections of cable iuli of oil at apredetermined pressure. The standpipes 7 and 8 lead upward to suitablylocated reservoirs l0l0 which are preferably collapsihis and expansibledrums performing the function of both reservoirs and expansion.

for the respective sections of one phase will receive oil from the cableor discharge oil into the cable at substantially the same rate as thoseof another phase for changes in temperature due either to atmosphericconditions or to variations in load transmitted by the line, but if thegroup of tanks for one phase should show an expansion or contractionexceeding that of another phase; heed should be given at once to thecondition of the cable, as such differential operation is indicative ofan abnormal condition.

Suppose for illustration that the tanks have been adjusted so that theamount of oil in the tanks under the condition of minimum temperature inthe winter time and without load is 25 gallons, and that thecorresponding amount for the condition of maximum load on the cables is50 gallons, then, if dependence should be placed upon a device whichwould act to give an indication only due to a predetermined minimumcontent of oil in the corresponding reservoir, the remaining oil undersuch conditions might escape or flow from the tanks before arrangementcould be made to replenish the supply. If, however, the signaling schemebe arranged to send in an alarm when the tanks connected to one phaseshowed an oil content five gallons less than one of the other is lower,but the reserve measured in time for a given lea-k to emptythe tank isnot greatly different.

I have provided means for determining the average condition of expansionor contraction by weighing the reservoirs or expansion tanks of eachsection for each phase and comparing the weight of one section with theweight of the expansion tanks of the corresponding section of anotherphase or other phases.

Each of the reservoirs 10-10 is suspended in a suitable mounting as, forexample, on a yoke 11 connected to a lever 12 pivoted at 13 andcounterbalanced by a spring 14. A counterbalancing weight might beemployed instead. To the lever 12 I have connected an expansible element15 by suitable connection 16, so that the expansible element 15 isactuated to change its displacement by movement of the lever 12. Theexpansible elements 15 are preferably small corrugated metal cylindersclosed at their upper ends and connected at their lower ends to tubes17, which tubes 17 in turn are connected to an oil filled manifold tube18.

Both of the cylinders 15 for a. given section are manifolded to the pipe18 and this pipe 18 is filled with a suitable fluid such,

for example, as oil of the same character as.

that employed in the cables.

The pipe 18 is preferably provided with an expansion tank 19 and with afilling and regulatin chamber 20. The chamber 20 is a collapsi le metaldrum, which may be expanded or contracted for zero adjustment as bymeans of the adjusting screw 21.

This Zero adjustment is particularly valuable under the followingcircumstances. Assume that a leak develops in a section of the cable andsome of the oil escapes from the cable and the reserve in the reservoirsconnected to that section is thereby decreased. The signal meansindicates the leakage and thereupon the workmen stop the leak bysuitable repairs. Now it may be unnecessary to fill the reservoirs againbecause there may be sulficientmemaining to permit operation to continuefor a time. The cost of replenishing the reservoirs is heavy because ofthe special processes and apparatus required even though only a smallamount of oil is to be introduced. Hence, by suitable manipulation ofthe adjusting screw 21 and the reservoir 20, the displacement of oil inthe transmitting tube 18 and connected parts may be compensated for thechange of contents in the corresponding reservoirs 1010 of thatparticular section.

The pipe 18 is connected to gauge element 22 which has a movable member23 like a steam gauge pointer, said movable element 23 being actuated asby means of a Bourdon tube or other element responsive to pressure ordisplacement and the movable element 23 plays between pairs of contacts24 and 25, mounted on a block 26. The block 26 is movable on a guide 27being held thereupon in any position by'frictional engagement.

The transmitting tube 18 and its connected parts form a closed system.The expansion tank 19 contains a body of air trapped in the top and,hence, displacement of liquid from the small bellows 1515 is permittedinto the tank 19 with a corresponding rise of pressure in the system towhich the gauge 22 is connected. Thus variations of displacement aretranslated into variations of pressure for moving the Bourdon tubes orlike elements in the indicators 22. Obviously, if the active element hadsuflicient change of capacity as might be provided by a bellows elementas shown in Figure 3 at 4040b and 400, the displacement of said bellowswould be a measure of the displacement caused in the elements 1515 byvariations of contents of reservoirs 10--10.

In Figure 1 I have shown the contacts 24 and so arranged as to close thecircuit 28 for tripping the signal sending device 29 upon the occurrenceof differences in pressure or displacement of the transmitting ormanifold tube 18 for one phase from that of an adjacent phase. Thecontacts 24-25' of a corresponding indicator (not shown) have been shownas connected to give such differential indication between two phases.The third phase is not shown in this figure, but by reference to Figure2 the complete arrangement will be seen.- As heretofore explained, Iprovide a differential indicating system where rise or fall of onesect-ion beyond the rise or fall of the corresponding sections gives theindication, as for example, by tripping the code sending device 29. Thecode sending device 29 is a non-interfering fire alarm box of knownconstruction connected in a circuit 29' to one or more supervisingstations where the code signal is received.

The diagram of Figure 1 shows two such signaling boxes for each tower,but it will be apparent that asingle box per tower may be arranged to betripped or operated by the indicating devices of both adjacent sections.The code signal may be received at more than one receiving orsupervising station.

In Figure 1 I have shown the instrument 22 for the A phase and indicatedthe contact springs 24 and 25 for another phase so that difierentialoperation is required between two gauges for two phases.

In practice the closing of circuit 28 is controlled by differentialaction between all three phases as will be explained more in detail inconnection with Figures 2 and 3. Ohviously it is possible to put all thecontacts 2425 for corresponding sections of each phase in parallel andallow a certain amount of play for each pointer 23 between such contactsprings, but that does not give the close supervision that can beobtained by the differential action hereafter illustrated and describedin detail.

Preferably, as shown in Figure 2, the gauges for corresponding sectionsof the three phases are connected to give an indication only if thepressure or displacement of one manifold tube 18 forone section isrelatively low, or relatively high, with respect to another. The mannerin which this is accomplished is by connecting the circuit of theresponsive magnet 30 through the pairs of springs 24 and 25 of each ofthe instruments so that the contact pair 24 on one instrument must beclosed and the contact pair 25 of another instrument must be closed atthe same time in order to complete the circuit. To this end all of theouter spriings of the contact pair 25 are connected in parallel with thewire 31 and the other contacts of the contact pair 24 are connected inparallel to the wire 32 and the inner springs of both pairs of eachinstrument are connected in parallel to an intermediate wire 33. Thusassume that in the instrument 22 in phase A the pointer 23 swingscounter-clockwise and closes the contact pair 25- and at the same timethe pointer 23b of the instrument 22b swings clockwise and closes itscontact pair 24?). It will be seen that a circuit is completed from thewire 31 to the contact pair 25 of the instrument 22 to the intermediatewire 33 to the inner contact of the pair 246 and to the outer spring ofthe pair 24?) and thence by wire 32 back through the magnet 30 to thesource of current 34. The magnet 30 is thereupon energized, attractingits armature 35 and closing a circuit 36 through an indicatinginstrument 37 which in the present instance has been shown as a simpleannunciator drop. Preferably the magnet 30 controls the tripping of acode sending box 29, as shown in Figure 1. @bviously, any form ofsignalling device which is desirable or suitable under the circumstancesmay be employed. In like manner if the pressure in any one of the threemanifold pipes should drop and the pressure in the others advance thecircuit will be closed giving the alarm.

I provide also limit contacts 66 and 67 arranged in the path of themovable block 26 to close the controlling circuit of the magnet 30, incase the pressure or displacement in the tube 18 of any particular phasedrops to a predetermined limit. In other words, if the pressure ordisplacement in any of the tubes 18 should drop to a predeterminedminimum an alarm is to be given since thiswould indicate leakage of theinsulating 011. In a similar manner the contacts 67 are arrangedindividually to give an alarm in case of excessive pressurecorresponding to a rise of temperature above apredetermined value in anyparticular phase.

Obviously, since the blocks 26 e connected together, only a single setof springs 66 or 67 is required.

It will be apparent that if the expansion of the three phases issubstantially the same then the-movable members 23 will swing, forexample, in a clockwise direction, and if this expansion is more thansuflicient to close the contact springs 24 of each of the indicatingdevices the mounting blocks 26 on which they are supported will beshifted on a guide or track 27 to the extent required.

Obviously, the mounting blocks 26 of three instruments may be connectedtogether as indicated at 68 so that continued advance of one of theindicator members 23 ahead of the others would serve first to close itscorresponding contact pair 24, and next to bring the contact pair 25 ofanother instrument into engagement with the corresponding pointer 23 toclose the circuit at such contact pair.

In Figures 3 and 4 I have indicated a modified form of the contactmechanism for performing the aforesaid functions.

In this case the three tubes 18 for the three phases are-connected toexpansible elements in the form of small metal bellows 4O mounted on acommon base 41. Each of the small bellows has a rod such as 42 adaptedto be moved by contraction and expansion of thecorresponding bellows.The upper ends of these rods are guided in a cross bar 43 extendingbetween the vertical guides 44. A common base board 45 is guided on thevertical guides 44 as by means of pairs of rollers 46 and 47 and thebase board 45 with the parts mounted thereupon is counterbalanced .so asto remain in any position on the guides is adapted to make electricalcontact with the stationary contact screws 54 or 55, these contactsbeing adjustable in their mountings on the baseboard 451 A pair of limitcontact springs 56 are mounted on the frame and are adapted to beengaged and closed by a pin 56' I on the baseboard 45 in case thebaseboard 45 tor drop.

moves down far enough to exceed a predetermined position. The positionof these contacts 56 is adjustable on the frame so that the said minimumposition may be suitably predetermined. In like manner limit springs 57may be employed to indicate when the baseboard 45 has moved up beyond apredetermined upper position. y

The movable contacts 53 of the three phases are all connected together,as indicated in Figure 4, and the lower contacts for the three phasesare all connected together in multiple by a wire 59 ,and the uppercontacts 54 are all connected together in multiple by wire 60. Themagnet is included in circuit with its source of current 61 and wires 60and 59. It controls in turn a circuit 36 through its armature foroperating a signal device 37, shown in this case as a simple annuncia-To close the circuit of the magnet 30 requires that the wire 62 whichconnects the 65 contacts 53 should complete a circuit between Thecounterbalance for this base nections.

the wires 60 and 59. This may be done either by dropping one of thecontacts 53 against its corresponding lower contact 55 and at the sametime raising one of the contacts 53 into contact with the correspondingupper contact 54 or a circuit may be established by one of the contacts53 rising or falling in advance of the other contacts or of anothercontact. This is true because continuous motion of one of the contacts53 bringing it into engagement with the screws 54 or 55 causes theentire base board to be moved up or down as the case may be, therebybringing one of the screws 54 or into engagement with another contact53, whereby the circuit for the magnet 30 will be completed.

The limit contacts 5657 may be connected to the wires and 59, as shownin Figure 4, or they may be connected in a separate circuit to give adistinctive signal.

For zero adjustment in case of a loss of oil from one of the sections,the adjustable collar 52 on stem 42 may be shifted.

'It is obvious from the above description that one of the sets ofcontacts 54-55 or 546-555 or 540550 may be omitted by providing abetween t e corresponding rod 42, 42?), or 420 and the base board 45 andmaking a cor-' responding change in the electric connections whereby acircuit is established between. the conductors 59 and 60 of Figure 4when either one of the two remaining contact arms engages either itsupper or its lower contact. This may be readily accomplished byconnecting both the upper and the lower contacts that are mounted on thebase board and engageable by the two remaining contact arms to the lowerconductor 59 instead of to the lower and upper conductors 59 and 60 asshown in Figure 4. The two remaining contact arms may then be connectedto the conductor 60. In like manner one of the sets of contacts such asthe contacts 246' and 25b ma%be omitted from the modification shown inigure 2 by providing a permanent me-' chanical connection between thepointer 23b and the block 266. If this were done then the contacts 24and 25 and the contacts 240 and 250 would be connected in arallel withthe contacts 66 and 67 instead 0 in the manner shown.

ermanent mechanical connection I From the above it will be seen that mysys- I tem of indication provides means by which the overall increase ordecrease of a particular group of reservoirs, as for example, for asingle, phase, may be ascertained. The means for algebraically addingtogether the increases or decreases of the reservoirs for the sectionsof a given phase is in the present case the fluid containing pipe 18 andits con- It' will be obvious that other specific means for performing,this integration may be provided as, for example, resistances in anelectric circuit may be employed. It

nesaees operation of said phase in absolute terms,

that is, the expansion or contraction of the oil in that particularphase without reference to what is going on in the other phases.Likewise, the position of the individual lever 12 indicates thecondition of the contents of the particular associated reservoir 10.

The present application is particularly directed to the indicating meansused, the electric system wherein the indicating means is usedconstituting the subject matter of my parent application of which thisapplication is a division. I have however described, in more or lessdetail, the electric system and the manner whereby the differentialindicating means is connected to the system in order to afford a clearunderstanding of the utility of my improved differential indicatingmeans. l/Vhile the specification describes one manner of using mypresent invention it is to be understood that the differentialindieating means herein shown is not limited in its use to a system suchas has here been de scribed but is of a more general application.

It is to be understood that my present invention is not limited to theprecise constructions herein set forth but is capable of variousmodifications within the spirit and scope of the appended claims. What Icon sider new and desire to secure by Letters Patent is:

1. A difl'erential indicating means comprising two movable elements,means for moving each of the elements in accordance with respectivevariables to be differentiated, and movable means extending between thetwo elements and normally out of engagement with each of them andmovable by either of them into operating engagement with the other..

2. A differential indicating means comprising two movable elements,means for moving each of the elements in accordance with respectivevariables to be differentiated, movable means extending between the twoelements and normally out of engagement with each of them and movable byeither of them into operating engagement with the other, and signallingmeans actuated responsive to the movement of the third element by one ofthe first elements into engagement with the other one of the two firstmentioned elements.

3. A differential pressure indicating means comprising two movableelements, means for moving each of the elements in accordance withrespective pressures to be differentiated, movable means extendingbetween the first two elements and normally out of operating engagementwith either of them and movable by either of them into engagement withthe other, and signalling means actuated responsive to the movement ofthe last named movable means by one of the first elements intoengagement with the other one of the two first mentioned elements.

4. In combination, a movable contact making member, a movable contactsupporting structure including a set of contacts engaged by the contactmaking member responsive to a predetermined movement of the member, saidstructure being movable with the contact making member upon continuedmovement of said member, a second movable contact making member, and asecond set of contacts mounted on the supporting structure and movedinto engagement with the second contact making member upon movement ofthe supporting structure, and alarm means controlled by the joint actionof both contact making members.

5. In combination, a movable contact making member, a movable contactsupporting structure including a set of contacts in the path of movementof and engageable by the contact making member, said structure beingmovable with the contact making member upon continued movement of saidmember after it engages a contact on the structure, a second movablecontact making member, and a second set of contacts mounted on thesupporting structure in the path of movement of the second contactmaking member and moved into engagement therewith upon movement of thesupporting structure, an electric circuit controlled by the joint actionof both contact making members, and means for controlling said circuitresponsive to the movement of the contact supporting structure beyondpredetermined limits.

6. In combination a set of responsive devices each having a movableelement and each having a movable contact supporting structure adaptedto be engaged by the movable meter element and moved in either direction thereby, and a connection between the movable contact supportingstructures of the respective devices for moving them in unison wherebywhen one of the structures is moved by its element the other structureis thereby moved.

7. In combination a set of responsive devices each having a movableelement and a movable contact supporting structure adapted to be engagedby the movable element and moved in either direction thereby; and aconnection between the movable contact supporting structures of therespective devices for moving them in unison whereby when one of thestructures is moved by its movable element the other structure isthereby moved, the contact supporting structure of each device includingsets of contacts engageable with the opposite sides of their respectivemovable meter elements. v

8. In combination a plurality of expansible and contractible members,movable rods actuated by the. respective members, and means fordetecting differences in the movement of the respective members, saidmeans comprising an arm connected to the respective rods, a 10 singlemovable structure adapted to be engaged by either side of any arm andmoved thereby, and indicating means actuated responsive to an engagementbetween the mova le structure and opposite sides of two different arms.7

9. Differential indicating means comprising a lurality of responsivedevices normally sub set to the same variable conditions and actuatedalike responsive to like variations in the respective conditions, one ofsaid devices including a member movable in response to said variationsand movable over a prescribed path, a member actuated by another of saiddevices and movable over the actuated alike responsive to likevariations in the respective conditions, one of said devices including amember movable in response to said variations and movable over aprescribed path, a member actuated byanother of said devices and movableover the same path, one

40 of said members including a contact making member, the other of saidmembers including a cooperating contacting member embracing the contactmaking member but spaced therefrom and in the path of the movementthere- }i-of, and limit switches controlled by at least one of said twofirst named devices.

, 11. In combination, a movable element movable over a prescribed path,a second movable element movable over the same path and including a pairof members spaced on opposite sides of the first element and in the pathof movement of the first element, means including a-responsive deviceresponsive to a predetermined type of variations for mov- 1,eus,ses

which the devices are responsive, whereby the relation between thecontact and the contact making member is unaltered upon like actuationof the devices and is changed responsive to unlike actuation of saiddevices.

In witness whereof, I hereunto subscribe my name this 15th day ofDecember, 1930. DENNEY W. ROPER.

SI ing the first element, and means including .a

responsive device responsive to the same type of variations for movingthe second element.

12. In combination two pressure responsive devices, means including acontact mem- 00 ber movable by one of the devices responsive to pressurevariations, means including a cooperatin contact makingmember movable bythe ot er device responsive to pressure variations, said members beingmovable alike 65. responsive to like variations in pressure to izo

